1. Field of the Invention
This disclosure relates to compositions and processes for improving brightness and optical properties, preventing loss of brightness, and enhancing resistance to thermal yellowing in pulp and paper manufacture. More particularly, this disclosure relates to compositions comprising borohydrides, which either alone, or in the presence of optical brightening agents, effectively enhance the brightness and optical properties of a paper product as well as increase its thermal stability.
2. Description of the Related Art
Pulps produced by either mechanical or chemical pulping methods possess a color that can range from dark brown to cream, depending on the wood type and defibering process used. The pulp is bleached to produce white paper products for a multiplicity of applications.
Bleaching is the removal or alteration of those light-absorbing substances found in unbleached pulp. In the bleaching of mechanical pulp, the object is to decolorize the pulp without solubilizing the lignin. Either reducing (e.g. sodium hydrosulfite) or oxidizing (e.g., hydrogen peroxide) bleaching agents are usually used. The bleaching is often a multistage process. The bleaching of chemical pulps is an extension of the delignification that started in the digestion stage. The bleaching is often a multistage process, which stages may include chlorine dioxide bleaching, oxygen-alkaline delignification, and peroxide bleaching.
Discoloration mostly ascribed to thermal aging, results in yellowing and brightness loss in various stages of papermaking processes employing bleached pulp, as well as in the resultant paper products. The industry significantly invests in chemicals, such as bleaching agents and optical brighteners, which improve optical properties of the finished paper or paper products. To date, however, the results have been less than satisfactory and the economic losses resulting from discoloration and yellowing present significant ongoing challenges to the industry. Accordingly, there remains a need for a successful and practical solution to prevent the loss of brightness and undesirable yellowing of pulp and paper.
Previous technology has been developed to improve and stabilize brightness, and enhance resistance to yellowing in the papermaking process (see U.S. Pat. No. 8,246,780, the contents of which are expressly incorporated into the present application by reference). In further development, the present inventors have discovered a way to significantly improve this technology based on unexpected synergism between a composition described in U.S. Pat. No. 8,246,780, and another reductive chemical, which is not taught or suggested in this reference.